Abstract

Tungsten oxide (WO3) nanorods array prepared using chemical vapor deposition techniques was studied.
The influence of oxygen gas concentration on the nanoscale tungsten oxide structure
was observed; it was responsible for the stoichiometric and morphology variation from
nanoscale particle to nanorods array. Experimental results also indicated that the
deposition temperature was highly related to the morphology; the chemical structure,
however, was stable. The evolution of the crystalline structure and surface morphology
was analyzed by scanning electron microscopy, Raman spectra and X-ray diffraction
approaches. The stoichiometric variation was indicated by energy dispersive X-ray
spectroscopy and X-ray photoelectron spectroscopy.

The nanostructured tungsten oxide material exhibited many excellent properties because
of their particular phase structure and huge surface areas, which depend greatly on
the experimental parameters. In previous experiment of chemical vapor deposition (CVD),
it was realized that several factors, such as filament temperature, electrical current,
gas flow and the composition of gas, would affect the structure of the sample. The
major factors could be the substrate temperature and the chamber pressure [8]. Moreover, the effect of the reaction gas concentration on the sample properties
was also preliminarily studied [9].

Based on the previous achievements, the focus of the present paper would be on two
issues: to analyze the influence of oxygen gas concentration (OGC) on the stoichiometry
phase, and to study an effect of substrate temperature on the crystalline structure
of tungsten oxide nanorods array. All samples have been characterized by using Raman
spectra, scanning electron microscopy (SEM); energy dispersive X-ray spectroscopy
(EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were also
employed to characterize the samples.

Experimental Set Up

The nanostructured tungsten oxide materials were synthesized using a CVD technique.
The Molybdenum (Mo) wafer was used as deposition substrate. Before placing the substrates
in the CVD chamber, the mirror-like surface of the polished substrates were ultrasonically
washed in a methanol solution for 5 min, rinsed with acetone, and dried with helium.
After placing the substrate, the chamber was pumped down to 2.0 × 10−5 Torr before feeding the gases. Two kinds of gas mixture, 8.7% of CH4, 0.3% O2, and 91% H2and 8.3% of CH4, 0.7% of O2, and 91% H2gases were used. The flow rate of mixed gases was 5SCCM. The gas pressure inside the
deposition chamber was maintained at 500 mTorr during the deposition. An AC power
supply with electric current of 10 A and voltage of 8 V was used to heat the tungsten
filament to temperature 2,400 °C to promote gas phase activation.

Results and Discussions

Figure 1 showed two SEM images of surfaces of the samples prepared under OGC of (a) 0.3% and
(b) 0.7% in mixture gases at 400 °C for 1 h of deposition. Differences between the
two surface structures were distinguishable. Generally, low OGC resulted in yielding
nanoscale WO particles, which was shown in Fig. 1a. The particles uniformly distributed. The scale of the particles was similar, which
was 1 μm around. The film’s color looked like ivory-white, and its surface appeared
glossy. Figure 1b showed the SEM image of the samples under high OGC. The sample’s color was violet
blue, which was the typical color of tungsten trioxide. The sample’s surface was dim.
It could be easily observed that the tungsten oxide rods arrayed very well. They were
vertical to the substrate. The diameter of the rod was 400 nm averagely. The number
of rods per unit area (Fig. 1b) was almost the same as the number of the total particles on the top surface of the
sample (Fig. 1a). Therefore, it could be assumed that the particles on the top layer in sample (a)
could be the base of the tungsten oxide rods shown in Fig. 1b. The particles on the bottom layer covered the substrate surface tightly, and then
the top layer provided the seeds of the rods. If the OGC were high enough as the precursor,
the tungsten oxide nanorods would keep yielding. If not, the nanorods would not exist.
As mentioned above, the gas pressures of both experiments were kept nearly constant
except slight variation of OGC from 0.3% to 0.7% in the gas mixture. Interest is that
so little variation of OGC resulted in completely different WO3 nanostructure.

Figure 1. SEM images of the samples prepared under (a) low OGC, and (b) high OGC

Figure 2a, b showed the EDS of two tungsten oxide samples (Fig. 1a, b). The element component quantitative result was presented in Table 1. The EDS pattern in Fig. 2 indicated the sample not only consisted of tungsten and oxygen, but also some carbon
atoms. The EDS signal (Fig. 2a) of oxygen was obviously weaker than that in Fig. 2b. This was in good agreement with the experimental conditions. From Table 1, it could be seen that variation of the oxygen concentration in the mixture gases
in the chamber from 0.3% to 0.7% yielded oxygen component up to nearly 7% and 51%,
respectively, inside the tungsten oxide samples. The ratio of atomic percentages between
tungsten and oxygen was almost 9 under low OGC. In this case the phase of tungsten
oxide supposed to be in sub-stoichiometry state. It could also be observed that the
composition of carbon was remarkable which was even 4 times more than oxygen. On the
other hand, under high OGC, the tungsten content decreased down to 38.66%, which was
20% lower than former one, whereas the component of oxygen largely increased up to
50%. Accordingly, the carbon component decreased rapidly, less than 20%.

Based on the data above, the stoichiometry variation can be given. As seen under low
OGC, the carbon content inside the sample was higher than that of oxygen. Therefore,
two possible chemical states might coexist inside the sample. One was tungsten oxide
together with tungsten carbide. Due to lack of oxide component, the stoichiometry
phase of tungsten oxide should be WO3−x, wherexwas related to the stoichiometry phase of tungsten carbide. The second possibility
was that carbon atoms, tungsten atoms, and tungsten oxide mixed but independently
existed, which mean there were no chemical bonds among them. This expectation has
been confirmed by XPS or XRD measurements below.

When the OGC in the mixture gases was high, the obtained oxygen content inside the
sample was up to 50%. The percentage of the carbon inside the sample descended to
10%. Consequently, tungsten oxide dominated the sample. This was verified by using
XPS. Figure 3a showed the tungsten peaks for the sample prepared under low OGC. Four peaks were
observed at 37.97 eV, 35.87 eV, 33.38 eV and 31.2 eV. The typical doublet W4f peaks
were clearly visible in the spectra, which were at 31.2 eV and 33.38 eV. The existence
of these two peaks strongly proved the assumption of deposition of atomic tungsten.
The two upper binding energy peaks exhibited the presence of oxygen modified W4f5/2 and W4f7/2 status. It was also found that there was a shoulder at upper energy side of each
atomic tungsten peak. By looking up the database of National Institute of Standard
Technology (NIST), these shoulders were related to WO2 and WOx. No specified carbon modified tungsten peaks could be found in the XPS profiles of
the samples. Considering the assumption of compound component mentioned above, it
was concluded that atomic carbon, atomic tungsten and sub-stoichiometry tungsten oxide
existed in this tungsten oxide particle-based thin film.

The XPS profile of sample (b) prepared under high OGC was shown in Fig. 3b. The oxygen modified tungsten features remained unchanged, indicating the presence
of stoichiometry tungsten oxide. The peaks of atomic tungsten vanished. Moreover,
the shoulder peaks related to WO2 or WOxalso disappeared. This evidence strongly supported the stoichiometry phase evolution
of the tungsten oxide.

Figure 4 showed the Raman spectra of the tungsten oxide samples related to Fig. 1a, b. Two broad but weak bands marked with K1 and K2 located in 770 cm−1 and 870 cm−1 which was shown in Fig. 4a, were related to tungsten oxide. Typical Raman peaks of crystalline tungsten oxide
located at 700 cm−1 and 800 cm−1. The shift resulted from the different chemical experimental conditions. In fact,
such weak humps revealed that the obtained tungsten oxide particle-based film was
in amorphous states.

Under high OGC, much more prominent Raman spectra peaks at 701 cm−1and 801 cm−1were indicated in Fig. 4b, which supported the existence of tungsten trioxide. A conclusion from Raman spectra
was revealed: the crystalline tungsten oxide has been yielded under high OGC. Variation
from the two weak humps in Fig. 4a to prominent peaks in Fig. 4b indicated that the crystalline structural evolution followed the change of OGC.

The XRD patterns of the samples were showed in Fig. 5. All four XRD peaks of the first sample shown in Fig. 5a were from tungsten components. From XPS, we have also known that the first sample
included atomic tungsten component, WO2 component, and WO3 component. These results provided evidence again that the first sample was in amorphous
status. It was in good agreement with the data obtained from Raman spectra. XRD pattern
of the second sample shown in Fig. 5b was complicated. It included atomic tungsten, WO2 and WO3 XRD peaks. WO3 XRD peaks dominated all spectral lines. The peaks marked as 002, 020 and 200, were
related to monoclinic tungsten trioxide. The peak of crystalline orientations of 020
was much stronger than that of 200 and 002 orientations. Similar phenomenon was observed
at the 2θ diffraction angles between 47o and 51o, which were associated with the orientations of 004, 040, and 400. Meanwhile, the
peak of 040 was the strongest one. This fact showed that the polycrystalline tungsten
trioxide of the sample was yielded, and the orientation of 020 dominated the trend
of growth.

In summary, the stoichiometry phase evolution of tungsten oxide highly depended on
the variation of OGC in mixture gases during deposition. Following an increase of
OGC, the sub-stoichiometry tungsten oxide-WO2and WOx-would become stoichiometry tungsten oxide. It was also found that atomic tungsten
and carbon without any chemical bond structure would be mixed inside the sample. The
variation of OGC also determined the structural evolutions from amorphous to crystal.
Low OGC caused yielding amorphous tungsten oxide, whereas high OGC resulted in producing
polycrystalline WO3.

As a comparison, the effect of variation of substrate temperature on the sample nanostructure
and chemical bond was also studied. Figure 6 shows SEM images of the samples prepared at substrate temperature of (a) 800 °C,
(b) 1,000 °C and (c) 1,200 °C. All other conditions such as OGC in the mixture gas,
gas pressure, gas flow rate, filament temperature and deposition duration were kept
same as the sample shown in Fig. 1b
.

The morphologies of these three samples were prominently different. The sample of
tungsten oxide prepared at 800 °C (Fig. 6a) was thin, sharp and short. Nanobundle was generated in this sample. The diameter
of single nanorod was around 200 nm, and the length was 2 μm. The nanobundle was so
compact that it tended to yield to larger nanorod. The sample yielded at 1,000°C was
shown in Fig. 6b. The hump-like nanostructured tungsten oxide was obtained. The diameter of the hump
at this temperature was 500 nm approximately. It could be assumed that the nanobundle
in Fig. 6a gathered then became a hump. This dynamic phenomenon was similar to the little drip
gathering to be a larger drip. Furthermore, the tungsten oxide nanorod was clearly
shown in Fig. 6c. The diameter of the nanorod was more than 500 nm, and the length was longer than
5 μm.

Based on these three samples, it was concluded that the growth rate of tungsten oxide
increased following the rising of temperature, resulting in that the sample diameter
was large and the length was long. Similar result was reported by Chi et al. [10] and Pal et al. [11]. It was especially mentioned that the root of each nanobundle was thinner than the
main body [10]. So it could be inferred that the thin nanobundle at lower temperature could be the
base of the sample prepared at higher temperature, which was similar to the condition
shown at former paragraph concerning the different OGC.

Figure 7 showed Raman profiles for these three samples (Fig. 6). Several Raman peaks were clearly visible, where peaks (signals) marked with J and
K in the Raman profile, respectively. No shift existed for all these three spectra,
which showed the structural stability of the samples at high temperature was very
well. In general, the bands situated at around 700 and 800 cm−1 could be assigned to W–O stretching model, whereas the bands situated at around 130
and 270 cm−1 were associated to W–O bending modes of monoclinic WO3[12] Miyakawa has shown that the Raman bands at 809 and 718 cm−1 were for monoclinic WO3 and did not change as a function of temperature (<500 °C), [13] indicating the formation of a highly stable monoclinic crystalline WO3. The present data indicated that the stretching mode would not change even the temperature
higher than 500 °C. However, the bending mode vanished at 1,200 °C. Therefore the
bending mode was not as stable as stretching mode and it depended on the substrate
temperature.

Figure 7. Raman profiles for samples at different deposition temperatures

Conclusion

In conclusion, the variation of the properties of tungsten oxide highly depended on
the OGC of the gas mixture. Slight rise of OGC from 0.3% to 0.7% in the mixture gas
during deposition resulted in large change of the oxygen quantitative component in
samples from 7% to 51%; the morphologies of the samples varied from particle-based
film to nanorods array, and the chemical phase developed from sub-stoichiometry phase
to stoichiometry. The crystalline structure also altered to crystalline tungsten oxide
from amorphous structure. The sample prepared under high substrate temperature (800
°C–1,200 °C) was also investigated. The diameter and length of the samples’ nanostructure
grew up by raising the substrate temperature. The evolution of morphology was prominent,
whereas the structure was quite stable. Based on the evidences above, it could be
concluded that the properties of tungsten oxide were highly sensitive to the experimental
parameters during deposition, especially the OGC in the gas mixture.

Acknowledgements

This work has been supported by NSF-EPSCoR and DoD grants. We would like to thank
Mr. William’s assistance of Raman measurements, Mr. Ortiz and Ms. Hernandez for SEM
and EDS measurements, Mr. Esteban for XPS measurements and Mr. Wu for XRD measurements.